In starting an electric motor, it is often necessary to generate a signal showing that the motor is running at full speed. Such a signal is often referred to as an up to speed signal. A continuing problem in the design of motor controllers is to obtain a reliable indication that the motor is running at full speed. Previous methods of generating an up to speed signal include detecting the in rush motor current, and then detecting the subsequent fall in motor current, and this method is exemplified by the disclosures of Kuntner, et al., U.S. Pat. No. 4,288,828, issued Sept. 8, 1981. However, under certain conditions with some motors, the inrush current does not fall sufficiently rapidly to produce a reliable up to speed indication.
It is important to have a reliable indication that the motor speed has reached full speed when the motor is soft started. A soft start of a motor is achieved by slowly changing the firing angle of thyristors controlling current flow to the motor so as to slowly increase the RMS voltage applied to the motor. If the motor reaches full speed before the voltage applied to the motor reaches line voltage, then the motor may become unstable and develop oscillations in its speed.